This invention relates to a process for the preparation of isometric magnetic iron oxides by the precipitation of aqueous solutions containing iron (II) salts with alkaline precipitating agents followed by oxidation at temperatures of from 20.degree. C. to 100.degree. C.
The earliest magnetic recording processes used magnetic sound recording media containing isometric carbonyl iron powder of .gamma.-Fe.sub.2 O.sub.3 pigment. For some time now, however, the use of isometric pigments has to a large extent been replaced by the use of needle-shaped pigments in the field of magnetic media. Needle-shaped pigments have a higher coercivity than isometric pigments by virtue of their anisotropy due to their form, and they generally have a high remanence if they are orientated in the direction of recording when the tape is being produced. For a long time, therefore, developments have been virtually confined to those processes which enable pigments with a controlled needle shape to be produced.
It is only in more recent times that the advantages of isotropic recording media have again been considered (EP-A 0,044,258 and EP-A 0,043,882) although such media were proposed as early as 1955 (U.S. Pat. No. 2,941,901). Isometric magnetic particles are isotropic in their behavior and therefore very suitable for the production of isotropic magnetic recording materials such as those required, for example, for data recording.
Other fields of application of isometric magnetic oxides include the magnetic one-component toners as used in modern electrostatic photocopying systems and the magnetic printing dyes for use in magnetic printers or for marking bank notes.
Isometric magnetic pigments require, like all pigments for magnetic recording processes, a manufacturing process which is technically simple to carry out and which enables the properties of the product to be accurately controlled.
Processes for the preparation of isometric magnetic iron oxide pigments are known, DE-PS 891,625, for example, proposes the oxidation of alkaline precipitates of iron (II) salts with nitrate under pressure. The application of pressure renders this process expensive and complicated. DE-AS 1,112,725 proposes the alkaline precipitation of iron (III) salts. The production of magnetic oxides, however, requires a hydrothermal treatment of the oxides followed by reduction with hydrogen at elevated temperature. DE-PS 944,427 proposes the precipitation of iron oxalate which must subsequently be decomposed by heat. This process is problematic for various reasons. Thus, oxalates have a very high tendency to form dust and are difficult to handle in the apparatus required for their decomposition.
DE-OS 2,612,798 proposes for the preparation of isometric iron oxides by the precipitation of iron (II) salts with alkaline precipitating agents and oxidation. With this process, however, it is only possible to produce magnetites in the form of coarse particles. Finely divided isometric megnetites are obtainable according to DE-OS 2,617,569 by the preparation of mixtures of iron (II) and iron (III) salts in proportions corresponding to the stoichiometry of the megnetite, followed by alkaline precipitation. This process requires the use of iron (III) salts which are not only expensive but, in the case of FeCl.sub.3, also corrosive. The pigments obtained by this prior art process are superparamagnetic and unsuitable for magnetic recording purposes.